His rehabilitation from off-season neck surgery has slowed and he will be listed as doubtful for Indianapolis' opener at Houston. If he can't play, the unretired Kerry Collins will get the start.

A dubious milestone for Peyton Manning could be a devastating millstone around the neck of the Indianapolis Colts.

The All-Pro quarterback, the league's only four-time most valuable player, is in serious jeopardy of missing Sunday's opener at Houston because he has not sufficiently recovered from off-season neck surgery. Entering his 14th season, he has never missed a start because of an injury.

In a statement released Monday, the Colts — who typically are highly secretive about injuries — said that Manning's rehabilitation has "slowed" and that the quarterback "continues to deal with a complicated neurological recovery, the end date of which is unpredictable."

Manning, 35, who did not participate in training camp, will not practice this week and will be listed as doubtful for Sunday's game, with the Colts continuing to update his status as the week goes on.

If Manning cannot play, the Colts will turn over the offense to 38-year-old Kerry Collins, who came out of retirement two weeks ago to sign with Indianapolis as an insurance policy.

Having started 208 regular-season games in a row (plus 19 more in the playoffs), Manning has more than double the consecutive regular-season starts of the No. 2 quarterback on that list — his younger brother, Eli, who has started 103 for the New York Giants.

"He's been an ironman; there's no other way to put it," Caldwell said Monday. "It's doubtful that he plays this week, but it takes a very unusual individual to have that streak."

Manning's situation took a turn for the worse over the weekend, the Colts said, when back soreness that originally was thought to be a normal reaction to rehabilitation prompted doctors to initiate more tests and consult with specialists around the country.

Manning, who in July signed a five-year, $90-million contract, could be dealing with much more than a week-to-week problem. According to one expert, it is not a good sign that the quarterback's recovery has slowed or — as ESPN has reported — his triceps strength has plateaued.

"If he has specific weakness in a muscle group, that means whatever intervention that he's had done was probably not completely adequate to address the problem that he has," said Dr. Ty Thaiyananthan, neurosurgeon and co-medical director of the Chapman Neurosurgical and Spine Institute in Orange. He has not examined Manning and addressed the situation only in generalities because he does not know details about the case.

Thaiyananthan said that patients sometimes need more invasive procedures to get beyond a plateau in muscle strength, and those types of operations — such as a cervical fusion or inserting an artificial disk — typically would end the career of a football player.

In their statement, the Colts said that they will report only on "concrete medical facts" and that "it serves no useful purpose to speculate about hypothetical outcomes."

A starting quarterback is vital to any team's success, but Manning might be the NFL's most irreplaceable player. He has missed precisely one snap in his pro career because of an injury, when he suffered a broken jaw against Miami in 2001. The Colts fumbled on the play that he missed, a turnover that set up the winning score for the Dolphins.

Counting training camp and the regular season, 41 quarterbacks have been beneath Manning on the depth chart since the Colts made him the No. 1 overall pick in 1998. Only five of those players ever took a regular-season snap for the franchise.

"They have a niche that they do, and they're letting No. 18 take it to whatever level he wants to," former Manning backup Mark Rypien said.

The club's success under Manning is one for the record books. In addition to winning a Super Bowl, the Colts have made the playoffs nine years in a row and boast the league's best regular-season record since 1999 (138-54). In 2009, they set a league record by becoming the first franchise to win 12 or more games in seven consecutive seasons.

So reliable and durable is Manning, the Colts haven't had to spend a lot of time grooming a backup to actually play. Curtis Painter took the majority of snaps in training camp, but the club scrambled to find a seasoned veteran when the possibility of being without Manning became more real.

The silver lining for the Colts is that Collins, the former Tennessee Titans quarterback, is very familiar with the division-rival Texans. In five starts against Houston, he led the Titans to victory three times while completing 88 of 157 passes for five touchdowns with four interceptions.

It stands to reason that the Colts would shrink the playbook for Collins, seeing as their offense is so reliant on Manning's being a traffic cop before the snap, changing plays and directing players this way and that.

Collins said he had a feeling when he signed that the situation might reach this point.

"I thought the first week might be a possibility with Peyton's status so uncertain," he said. "There's no way I can replace someone like Peyton and what he means for this team and this franchise. But I will bust my butt to get ready, and I hope the guys see that."